The present invention relates to an elastic fixing roll which, being favorably applicable as a fixing roll in an electrophotographic copying machine, as a roll in a printing machine, or as a platen roll in a printer, and so forth, is capable of achieving an excellent release property.
For the rolls, platen rolls, or the like in printing machines, urethane rubber, EP rubber, silicone rubber, and so forth have been conventionally used, and silicon rubber rolls, fluororubber rolls, etc. are known to be in use as the fixing rolls in electrophotographic copying machines. However, as these types of rolls fail to achieve any sufficient release property even when they are used together with toners having features favorable to the release of paper from the rolls, a non-elastic roll covered with fluororesin, an elastic roll with a shrink-fit fluororesin tube covering its surface, or the like have been proposed. Also, another type of roll hitherto proposed is an elastic roll with a layer of fluororesin powder on its surface by coating a mixture of fluororubber and fluororesin powder thereon and baking the mixture.
Moreover, it has been in practice to coat release oil, generally, silicone oil, on the surface of the fixing roll in the process of the fixing operation in an electrophotographic copying machine, thereby improving the release property of the fixing roll. In such a case, however, it is necessary to prevent the fixing roll from swelling, which occurs as the result of the permeation of the silicone oil into the inside of the roll. For this purpose, such fixing rolls as an elastic roll with its surface covered with a shrink-fit fluororesin tube or the like and a roll (Japanese Patent Laid Open No. 205188/1989) made by forming a silicone rubber layer, a fluororubber layer or a fluoro-silicone rubber layer, and a silicone rubber layer in the stated order on a metal core bar have hitherto been proposed.
In the meanwhile, Japanese Patent Laid Open No. 285839/1987 discloses an elastic roll which the present inventors proposed. This elastic roll is made by soaking heat-resistant elastomer material in fibrillated polytetrafluoroethylene, particularly, extensible porous polytetrafluoroethylene, and joining these and thereby forming a covering layer on a metal core bar.
Now, the rolls, platen rolls, etc. which are made of urethane rubber, EP rubber, silicone rubber, or the like for use in printing machines, are not necessarily favorable in view of release property, though they offer favorable elasticity, so that such rolls have caused problems that they are liable to troubles such as the sticking of paper pieces to the roll and the winding of paper around the roll.
Also, particularly among the fixing rolls for electrophotographic copying machines, non-elastic rolls covered with fluororesin have the shortcoming that they lack in elasticity, and an elastic roll with its surface covered with a shrink-fit fluororesin tube is not necessarily satisfactory in terms of the elasticity of its surface because such a roll has small extensibility as the fluororesin is hard. Moreover, a roll with a layer of fluororesin powder formed on its surface achieves a favorable level both in its elasticity and its release property in the initial period following its installation, but the service life of the roll in respect of its release property is short since the fluororesin powder on its surface is liable to separate and to fall off from the roll.
Also, a roll with a silicone rubber layer formed over a fluororubber layer or a fluoro-silicone rubber layer, as the roll described in Japanese Patent Laid Open No. 205188/1989, is liable to the problem that since the silicon rubber layer on the surface of the roll does not have sufficient strength, the roll suffers for decline in the release property if the amount of the filling agent is increased in order to increase the strength of the surface layer, and also that since the roll does not have any sufficient adhesive strength between the silicone rubber layer and the fluororubber layer or the fluoro-silicone rubber layer, repeated fixing operations with repeated applications of silicone oil result in the growth of cracks in the silicone rubber layer on the surface, causing the separation of the surface layer in the worst case.
On the other hand, the elastic roll disclosed in Japanese Patent Laid Open No. 285839/1987 achieves very good release property and also realizes favorable affinity with silicone oil and favorable resistance to swelling, but is poor in its elasticity characteristics and inferior particularly in its elasticity recovering power. Additionally, the roll has another problem in that it is poor in its thermal conductivity, so that the surface temperature of the roll is lowered particularly in continuous copying operations.